STEMS AND THEIR VARIETIES 53 



and woody stems, as it is a matter of degree of development 

 of the wood within them : a wall-iiower or a rose, for example, 

 may be soft and herbaceous in its upper parts and hard and 

 woody below. 



Trees and Shrubs have well-developed woody stems, the 

 former possessing a single main stem or trunk, which is devoid 

 of branches for some distance above the ground; the latter 

 have no very distinct main stem, and the chief branches are 

 all much the same in thickness and spring from a point either 

 on or close to the ground. 



Many plants have stems which are too weak to maintain an 

 erect position ; they consequently grow along the surface of 

 the soil. Some plants have weak stems which always remain 

 prostrate, while others, designated climbing plants, have stems 

 which, although too weak to stand upright of themselves, are 

 nevertheless able to use suitable objects near them as supports. 

 Climbing plants support themselves in various ways. In ivy, 

 adventitious roots are developed on one side of the stem, and 

 these serve to fix the plant to bark of trees, walls, and rocks. 



Tropseoluras of gardens and wild clematis are supported by 

 their leaves, the petioles of which curve round the stronger 

 branches of plants growing near them. 



Peas and vetches are also enabled to climb by means of their 

 leaves, some of the leaflets of which are modified into thin thread- 

 like structures termed tendrils. The latter are sensitive to contact, 

 and wind round any slender object which they touch. Plants 

 such as the blackberry, rose, &c., are supported by means of 

 their stiff prickles. 



In twining plants the whole stem upholds itself by twisting 

 round neighbouring objects. The stems of some of them 

 always twine to the right when growing round a support; the 

 hop is an example : others, such as bindweed, twine to the 

 left. 



17. A number of peculiar modifications of shoots are met with. 



